Radio frequency identification (RFID) systems typically include at least one reader and a plurality of transponders, which are commonly termed credentials, cards, tags, or the like. The transponder may be an active or passive radio frequency communication device which is directly attached to or embedded in an article to be identified or otherwise characterized by the reader. Alternatively, the transponder may be embedded in a portable substrate, such as a card, tag, or the like, carried by a person or an article to be identified or otherwise characterized by the reader. An active transponder is powered up by its own internal power supply, such as a battery, which provides the operating power for the transponder circuitry. In contrast, a passive transponder is characterized as being dependent on the reader for its power. The reader “excites” or powers up the passive transponder by transmitting excitation signals of a given frequency into the space surrounding the reader, which are received by the transponder and provide the operating power for the circuitry of the recipient transponder. The frequency of the excitation signals preferably corresponds to the frequency of data signals communicated between the transponder and reader.
Once a passive transponder is powered up, the transponder communicates information, such as identity data or other characterizing data stored in the memory of the transponder, to the reader. The transponder communicates with the reader in a contactless manner by generating transponder data signals utilizing internal circuitry which typically includes a resonant LC pair made up inter alia of a capacitor and an antenna. The transponder data signals are characterized by a specific carrier frequency which is a function of the transponder LC pair. In particular, the transponder LC pair is tuned to a desired resonant frequency so that the transponder data signals generated thereby have a carrier frequency corresponding to the tuned resonant frequency of the transponder LC pair.
For example, transponders of the type conventionally termed proximity cards or proximity tags have an LC pair tuned to a resonant frequency range of 100 to 150 kHz, which enables the proximity card to generate transponder data signals at a carrier frequency within this same range of 100 to 150 kHz. This carrier frequency range is nominally referred to herein as 125 kHz carrier frequency and is deemed a low frequency. In contrast, transponders of the type conventionally termed smart cards have an LC pair tuned to a higher resonant frequency of about 13.56 MHz, which enables the smart card to generate transponder data signals at the same carrier frequency of 13.56 MHz.
The transponder data signals are transmitted in the form of electromagnetic oscillations into the surrounding space in which the reader resides via the antenna of the transponder LC pair. The reader contains its own internal circuitry including an LC pair made up inter alia of a capacitor and an antenna which receives and “reads” the transponder data signals (i.e., extracts the data from the transponder data signals) when the reader LC pair is tuned to essentially the same resonant frequency as the tuned transponder LC pair and correspondingly to the carrier frequency of the transponder data signal.
The excitation signal generating and transmitting functions and the transponder data signal receiving and reading functions performed by the reader as described above define a reader operating state termed a “data transaction state.” The data transaction state further encompasses reader data signal generating and transmitting functions, wherein information stored in the reader memory or otherwise generated by the reader is communicated to the transponder. The manner in which the reader communicates information to the transponder is essentially the same or similar to the manner in which the transponder communicates information to the reader. As such, the reader data signals are characterized by essentially the same carrier frequency as the transponder data signals.
Although a reader can continuously operate in the data transaction state, the functions performed by the reader while in the data transaction state typically have a relatively high power demand, which can rapidly deplete the power supply of the reader. This condition is particularly undesirable when the reader is powered by a self-contained portable power supply, such as a small disposable or rechargeable battery, which has a finite life. It is generally more power efficient to operate the reader in the data transaction state only when a transponder is within the read range of the reader, while operating the reader in an alternate state having a relatively lower power demand at all other times. A preferred alternate lower power reader operating state is termed a “detection state,” which is commonly enabled by a ring signal generator circuit and a transponder detection circuit provided within the reader. The reader operates continuously in the detection state except when the transponder detection circuit detects a transponder within the read range of the reader. The reader switches to the data transaction state upon detection of a transponder, but only for a limited time sufficient to complete communication between the reader and transponder before switching back to the detection state.
U.S. Pat. No. 6,476,708 to Johnson (the '708 patent), which is incorporated herein by reference, discloses an exemplary reader having a low power detection state and a high power data transaction state of operation. The reader includes a signal generator circuit which alternately acts as the ring signal generator circuit or an excitation signal generator circuit depending on the operating state of the reader at any given time. The reader further includes a small portable battery power supply and the transponder detection circuit which is coupled to the signal generator circuit.
The operating principle of the detection state is to detect a transponder within the read range of the reader by measuring changes in an impulse response on the reader antenna. The detection state is initiated by generating a detection pulse using the signal generator circuit and applying the detection impulse to the reader antenna. The detection impulse causes the reader antenna to transmit a ring signal into the surrounding space, which has a frequency corresponding to the resonant frequency of the tuned LC pair of the reader. The resulting ring signal causes a predictable impulse response on the reader antenna. Although the ring signal has insufficient to power to operate any transponders residing in the surrounding space, if a transponder having a resonant frequency at or near the resonant frequency of the reader is sufficiently proximal to the reader, the impulse response on the reader antenna is altered in a characteristic manner. In particular, inductive coupling of the reader antenna to the nearby transponder antenna causes a change in the impulse response on the reader antenna.
The reader employs the transponder detection circuit to detect this change in the impulse response. In particular, the transponder detection circuit monitors the level of a designated transponder detection parameter of the impulse response. When the transponder detection parameter reaches a predetermined threshold level, the presence of a transponder in the surrounding space is confirmed and the transponder detection circuit switches the signal generator circuit from the low power detection state to the high power data transaction state thereby terminating generation of the ring signals. As such, the signal generator circuit transitions to an excitation signal generator circuit, wherein the signal generator circuit draws increased electrical current from the reader power supply to generate and transmit an excitation signal which is sufficient to activate the transponder. The excitation signal is received by the transponder and powers the transponder circuitry, which in turn generates a transponder data signal for transmission to the reader. After the reader reads the received transponder data signal, the signal generator circuit switches back to the detection state and resumes generation of the ring signals while terminating generation of the excitation signals.
Since only ring signals are transmitted by the reader in the detection state, the reader runs at a very low duty cycle, yet at a high repetition rate while in the detection state. Consequently, the above-described technique enables the reader to operate with very low average power consumption to avoid accelerated dissipation of the reader power supply while maintaining a rapid response time for transponder detection.
The sensitivity, and correspondingly the detection range, of the reader in the detection state is highly dependent on closely matching the tuned resonant frequencies of the reader and transponder LC pairs. However, the entire population of transponders in a given RFID system is not always tuned to the same single resonant frequency. Instead a given population of transponders may exhibit a distribution of multiple resonant frequencies. For example, different manufacturers of transponders can elect to tune their transponders to different nominal resonant frequencies resulting in commercially available transponders operating at different frequencies. Therefore, it is desirable to provide a transponder detector for a reader which is capable of detecting transponders tuned to different resonant frequencies.
Accordingly, it is generally an object of the present invention to provide a reader which can selectively generate detection signals on a single reader antenna with different detection signal frequencies. It is generally another object of the present invention to provide a reader which can utilize the different frequency detection signals in a searching pattern for transponders tuned to corresponding frequencies. It is another object of the present invention to provide a reader which generates different frequency detection signals while operating in a state of very low power consumption. More particularly, it is an object of the present invention to provide a reader which transitions between generation of the different frequency detection signals without excessive power consumption. It is a further object of the present invention to provide a reader having a detection circuit which maintains a high circuit Q in order to maintain sensitivity regardless of which detection frequency is generated. It is another object of the present invention to provide a reader having active circuits and switching elements for the detection state of operation which are implemented within an integrated circuit utilizing a standard process such as a digital or mixed signal CMOS integrated circuit. It is yet another object of the present invention to provide a detection signal generator circuit which can be readily integrated with an existing conventional low frequency or high frequency reader or reader/writer. These objects and others are accomplished in accordance with the invention described hereafter.